1. Field of the Invention
Embodiments of the present invention relate to a temperature measuring system. In particular, they relate to a temperature measuring system for use within a gas turbine engine.
2. Description of the Related Art
A gas turbine engine is a heat engine using air as a working fluid to provide thrust. Air at atmospheric pressure enters the engine at an air intake and is compressed by the compressor blades. Compressed air then enters a combustion chamber where heat is added to the air by introducing and burning fuel at constant pressure, thereby increasing the volume of the air considerably. Gases resulting from combustion expand through the turbine, driving the turbine blades, and subsequently flow out of the exhaust.
The amount of fuel added to the air in the combustion chamber depends on the temperature rise required. The maximum temperature is, however, limited by the materials from which the nozzles and turbine blades are made.
The nozzles and turbine blades are built using materials that are able to resist heat and are not susceptible to thermal shock at high temperatures, such as nickel based alloys. Despite this, for a particular blade material there is an acceptable safe life and a maximum permissible turbine entry temperature. It therefore follows that it would be useful to have an accurate estimate of the temperature of the turbine blades, in order to ensure that the maximum turbine entry temperature is not being exceeded.
Optical pyrometers are often used in industrial applications to estimate temperatures, particularly when it is difficult to situate a temperature measuring device close to the object being measured. An optical pyrometer provides an estimate of the temperature of an object by focusing the radiation emanating from the object using optical lenses towards a radiation detector. The temperature of the object can then be estimated from the measured intensity of the light using, for example, the well-known Planck radiation law.
Optical pyrometers can provide a reasonably accurate estimate of the temperature in many situations. However, their use in a gas turbine engine can be problematic because the temperatures reached within the engine are very high. The lenses used often have a different thermal expansion coefficient to their housing, which has to be accounted for. The filler material used to connect the lenses to their housing may also susceptible to degradation at high temperatures. Furthermore, the optical lenses can become covered with soot and dirt while the engine is in operation. These factors all have a bearing on the accuracy of the temperature estimate provided by the pyrometer and can mean that it is less accurate than it would otherwise be.